FIG. 14 is a perspective view showing a conventional semiconductor laser device disclosed in, for example "MITSUBISHI DENKI GIHO", Vol. 60, No. 12, pp. 27 to 31, 1986. In FIG. 14, reference numeral 1 designates a heat sink formed of silver or diamond. A submount 2 formed of crystalline Si is put on the heat sink 1. Gold films 3 are formed on upper and lower surfaces of the submount 2. A semiconductor laser chip 6 is put on the submount 2. A p side electrode or an n side electrode 4 and an n side electrode or a p side electrode 5 are formed on lower and upper surfaces of the laser chip 6, respectively. Especially, the surface of the electrode 4 mounted on to the submount 2 is plated with gold. A wire 7 is bonded to the electrode 5.
Next, description will be given of a method for reducing stress applied to the semiconductor laser chip in the prior art. Since the heat sink 1 is formed of metal and the laser chip 6 is formed of a semiconductor, there is a large difference between their thermal expansion coefficients. For example, when the heat sink 1 is formed of silver and the laser chip 6 is formed of GaAs, the thermal expansion coefficients of the heat sink and the laser chip are approximately 19.5.times.10.sup.-6 /.degree. C. and 6.5.times.10.sup.-6 /.degree. C., respectively. In general, the laser chip 6 is attached to the heat sink 1 using an appropriate soldering material at a high temperature. When the high temperature falls to room temperature (low temperature), stress is applied to the laser chip 6 because of the above-described difference in thermal expansion coefficients, with the result that a laser characteristics are degraded. In order to avoid this problem, the submount 2 formed of Si is inserted between the heat sink 1 and the laser chip 6 in the prior art. Since the thermal expansion coefficient of the Si is relatively close to that of GaAs and the submount 2 formed of the Si has an appropriate thickness (approximately 150 microns), the stress generated by the difference in thermal expansion coefficients is reduced because of the submount 2, so that the stress is not applied to the laser chip 6.
In addition, there is another method for reducing the stress applied to the laser chip 6, in which the laser chip 6 is bonded to the heat sink 1 using a soldering material having a relatively low melting point (for example InPb).
As described above, according to the conventional semiconductor laser, since Si having a thickness of approximately 150 microns is inserted between the heat sink and the laser chip as a submount, heat generated in the laser chip is poorly conducted. Therefore, that mounting is not suitable for high power output and high current injection operation.
In addition, according to the method using a soldering material having a low melting point, a wire has to be attached at a lower temperature after the soldering step so the wire is not reliably attached.